Vesicular trafficking is crucial to cell function and survival, and deregulation of vesicular trafficking is associated with many human diseases, including neurodegenerative disease and peripheral neuropathy. The long-term goal of this research is to understand, at the molecular level, how vesicular trafficking is controlled in norma physiology and how this process becomes dysregulated in disease states. Charcot- Marie-Tooth disease (CMT) is the most common hereditary peripheral neuropathy that has been known for more than 125 years, yet the pathogenic mechanisms underlying this disease remain unclear and there are currently no effective treatment for peripheral neuropathy. Human genetic studies reveal that missense mutations in SIMPLE, a ubiquitously expressed protein of unknown function, cause an autosomal dominant form of peripheral neuropathy known as CMT type 1C. The connection of SIMPLE to CMT underscores the importance of understanding the function of this understudied protein and opens up new avenues for investigating the molecular mechanisms that trigger peripheral neuropathy. Recently, the applicant's team found that SIMPLE is an early endosomal membrane protein involved in regulation of endosome-to- lysosome trafficking and generated the first CMT1C mouse model expressing a disease-linked human SIMPLE mutant protein. In this project, the applicant's team will build on their recent studies and use a combination of biochemical, molecular biological, cell biological, proteomic, and mouse genetic approaches to define the mechanisms of SIMPLE action in regulating vesicular trafficking and elucidate the pathogenic pathways by which SIMPLE mutations cause peripheral neuropathy. Successful completion of this project will advance our knowledge of the fundamental mechanisms governing vesicular trafficking and provide useful information needed for development of effective therapies to treat peripheral neuropathy and other human diseases with disturbed vesicular trafficking.